Conventional aircrafts have a vertical tail and rudder, and they respectively have the functions of providing stability (tail) and control (rudder) around the directional axis (vertical) of the aircraft. The rudder should be designed to provide a suitable control capacity for normal piloting tasks in the flight envelope of the aircraft, and those include tasks like coordination of curves to execution of take-offs and landings in strips with cross-winds.
Multi-engined aircrafts usually don't have, in general, all engines directly installed on the symmetry plan of the aircraft, and therefore when the traction forces produced by each engine are not equal—in other words, when the traction is asymmetric—a moment of deflection (binary) acts on the aircraft due to the product of the traction asymmetry by the lever arm of that force. That binary should be compensated (counter-balanced) by the rudder to allow the aircraft to maintain a straight flight in the event of partial or total stop of one or more engines, as well as to allow appropriate control for maneuvers and landing in the sequence of such event.
Usually, the rudder is commanded by the pilot through pedals located in the cockpit. The action of the pedals can be mechanical, through cables and rods, or servo-mechanisms using an auxiliary power source, usually hydraulics, to operate. That option is more complex, heavy and expensive if compared with the a mechanical action architecture, and its use is justified by subjects of loads for surface action or aerodynamic cleaning, or by requirements of functional order.
Another important aspect related to primary flight commands is the reaction force of the pedals to commands (movements) executed by the pilot. The action systems are classified like reversible, when the aerodynamic load on the command surface is used to provide the reaction force to the pedal, and irreversible, when there is not feedback of the aerodynamic load for the pilot—in that case, the forces on the pedals are provided by artificial means such as springs and shock absorbers. Systems acted with aid of servo-mechanisms can have reversible or irreversible characteristics, while the mechanical systems are naturally reversible.
Classically, there are two alternatives when excessive pedal forces are found in the design of a mechanical (reversible) rudder control system; they are:                To substitute the mechanical (reversible) action system for a irreversible action system;        To use aerodynamic balancing to reduce the total aerodynamic moment that acts on the articulation axis of the rudder, thus reducing the pilot's work when deflecting the rudder in flight.        
The aerodynamic balancing is made through modifications in the rudder geometry and/or adoption of auxiliary aerodynamic devices, such as auxiliary surfaces as for example “tabs”. However, when the demand for reduction of forces is very high, it can happen that the aerodynamic balancing solution presents “overbalance” characteristics (i.e. excessive balancing) of the rudder, a term used to designate two phenomena:                inversion in the sense of application of the resulting articulation moment of the control surface;        reversion of the natural tendency of return of the pedal to neutral position in certain maneuvers, due the combination of low directional stability with great “fluttering” (tendency of the rudder in joining with the flight direction) of the rudder in skiddings.        
An usual solution for rudders of multi-engined aircrafts having reversible rudders is the use of a concept system known as “rudder bias” (rudder automatic compensation), described in the british patent GB 1,086,161. In this embodiment, the rudder is mechanically controlled and it is aerodynamically balanced in a such way to provide normal operation (when the traction is symmetrical) with appropriate pedal forces. When a traction asymmetry is detected, an auxiliary action system installed on the rudder mechanism is operated and it complements the pilot's action to control the aircraft. The patent originally claims a pneumatic action system operated by a pressure differential between gas pressure tubes installed in both engines; a modern form of embodiment is usually based on electric or hydraulic actuators and on electronic detection of the traction asymmetry.
However, it is an active system, having authority for autonomous deflection of the rudder, and the system failures can seriously affect the flight safety, just like an improper activation of the system or rudder command to move to the contrary side to the necessary for correction of the flight path of the aircraft.
It can be said that the motivation to find an alternative solution to the “rudder bias” system is to develop a simpler, light, cheap and mainly safe system that solves the mentioned problems. The solution is the adoption of a concept of passive protection system, instead an active control system for the command surface, resulting in a simpler, safe and cheap system.